Magnetic labelling of living cells creates opportunities for numerous biomedical applications, from individual cell manipulation to MRI tracking. Here we describe a non-specific labelling method based on anionic magnetic nanoparticles (AMNPs). These particles first adsorb electrostatically to the outer membrane before being internalized within endosomes. We compared the labelling mechanism, uptake efficiency and biocompatibility with 14 different cell types, including adult cells, progenitor cells, immune cells and tumour cells. A single model was found to describe cell/nanoparticle interactions and to predict uptake efficiency by all the cell types. The potential impact of the AMNP label on cell functions, in vitro and in vivo, is discussed according to cellular specificities. We also show that the same label provides sufficient magnetization for MRI detection and distal manipulation.